Mass production of ophthalmic lenses, in particular of contact lenses such as soft contact lenses, is usually performed in a fully automated process. In an embodiment of this fully automated process the soft contact lenses are manufactured with the aid of reusable molds comprising male and female molds. In one processing station, a lens forming material is introduced into the female mold. Lens forming materials that may be useful in such process are, for example, lens forming materials based on polyvinyl alcohols (PVA) or lens forming materials based on silicone hydrogels (SiHy). After introduction of the lens forming material into the female mold, the female mold is mated in a subsequent processing station with a corresponding male mold to form a mold cavity defining the shape of the soft contact lens to be formed. In a yet further subsequent processing station, the lens forming material within the mold cavity is polymerized and/or cross-linked, for example by irradiation with ultraviolet radiation (UV-radiation) to form the soft contact lens.
The polymerization/cross-linking process by irradiation with UV-radiation may be performed using molds in which the mold cavity of the mated male and female molds is not completely sealed but is connected by a small annular gap to a reservoir surrounding the mold cavity. During the polymerization/cross-linking process, excess lens forming material contained in the reservoir surrounding the mold cavity is allowed to flow from the reservoir through the small annular gap into the mold cavity to compensate for a shrinkage in volume which may occur during polymerization/cross-linking of the lens forming material with the aid of UV-radiation. An annular metallic mask may be arranged at the radially outer boundary of the molding surface of the male mold.
The annular metallic mask, in particular a chromium mask which may have been applied to the male mold through deposition of a chromium coating, shields those portions of the lens forming material which are not to be exposed to UV-radiation, so that the mask accurately delimits the space where the lens forming material enclosed between the mated male and female molds is exposed to UV-radiation to allow for a precise definition of the geometry of the lens edge of the soft contact lens to be formed.
After exposure to UV-radiation, the mold is opened through separation of the male and female molds, the soft contact lens formed is removed from the male or female mold, respectively, and may subsequently be transported to additional processing stations.
Once the contact lens has been removed from the male or female mold, the molds used in the production of the soft contact lens are cleaned and rinsed through exposure to cleaning and rinsing liquid jets. Cleaning and rinsing liquids comprise, for example, water or water mixed with organic solvents to remove from the used mold halves any debris such as residual lens forming material adhering to the mold, as well as deionized water for a final rinsing step. Thereafter, the cleaned and rinsed molds are dried in order to be ready for use in the next production cycle.
As has been outlined above, mass production of ophthalmic lenses includes the use of reusable molds. The reusable molds, or at least those parts thereof comprising the molding surfaces, are typically made from optically finished glass, for example quartz glass. Such reusable molds are particularly advantageous since they are transparent to UV-radiation, they are durable, they can be cleaned and rinsed after molding of a soft contact lens for subsequent re-use, etc. Due to the optical quality of the molding surfaces and the required accuracy of the chromium mask defining the geometry of the lens edge of the soft contact lens which is important for the wearing comfort, these molds or mold halves are rather expensive.
As has been indicated, such molds are used in mass production of soft contact lenses so that they are used in very high numbers of subsequent production cycles before they are replaced. However, after such a high number of production cycles it may occur that the chromium mask may slowly be degraded by the high number of cleaning and rinsing steps in which the chromium mask is exposed to the jets of deionized water. After a certain number of production cycles, it may occur that the mask is degraded to an extent that the accuracy of the mask is no longer sufficient to maintain the high level of accuracy required for the lens edge, so that the mold has to be replaced.
Due to the molds or mold halves being rather expensive, it is an object of the invention to increase the number of production cycles the molds can stand while maintaining the high level of accuracy required for the lens edge, and at the same time the molding surface of the mold halves must be reliably cleaned and rinsed before it is re-used.